1. Field of the Invention
This invention relates to heat bondable heterofilaments and to nonwoven fabrics made therefrom.
2. Description of the Prior Art
The use of heat bondable heterofilaments in the manufacture of nonwovens is well known in the prior art. Generally, such heterofilaments comprise two thermoplastic materials which are arranged in either side-by-side or sheath/core relationship with the two materials being coextensive along the length of the filament. One of the thermoplastics, a so-called latent adhesive, is selected so that its melting point is significantly lower than that of the other in the filament, and, by the application of heat and subsequent cooling, this component is made to become adhesive and bond to other fibers in the nonwoven. Such adhesion can take place either between like heterofilaments or between heterofilaments and conventional non-bonding filaments if these are also present in the nonwoven. The other component serves as a structural or backbone member of the fiber.
Although heat bondable heterofilaments were developed for use primarily in the production of light weight nonwovens, that is, nonwovens having relatively little weight per unit of area, they have achieved a somewhat limited commercial success in this area due to a number of deficiencies which are present in state of the art fibers. Foremost among these deficiencies are excessive shrinkage during thermal bonding, which leads to fabrics having an uneven density and non-uniformity of thickness; insufficient fiber-to-fiber bond strength, which leads to poor fabric tensile strength, as well as the production of nonwoven fabrics which are relatively lacking in such traditionally desirable textile qualities as drape, liveliness and bulk or loft.
Admittedly, an attempt has been made in the prior art to deal with the above-mentioned deficiencies. Tomioka, in an article entitled "Thermobonding Fibers for Nonwovens", Nonwovens Industry, May 1981, pp. 23-31, describes the properties of ES Fiber, a bicomponent material commercially available from Chisso Corporation of Osaka, Japan. This fiber, which comprises polyethylene and polypropylene in a so-called modified "side-by-side" arrangement (actually a highly eccentric sheath/core), is also, presumably, disclosed in U.S. Pat. No. 4,189,338, to Ejima et al. and assigned to Chisso Corporation. Among the attributes of this fiber, Tomioka deals most extensively with the relatively low thermal shrinkage which the fiber experiences during the thermal bonding step, and goes on to note that this property results in nonwovens which possess good uniformity of density and thickness, as well as good bulk, hand and drape.
While it is certainly the case that the fiber described by Tomioka represents a substantial improvement in the state of the heat-bondable fiber art to date, this prior art fiber nonetheless suffers from several shortcomings. For example, while the fiber does indeed exhibit an amount of thermal shrinkage which is less than that of earlier fibers, it can be demonstrated that the fiber nevertheless still shrinks to a substantial and undesirable degree. Furthermore, although the elimination of thermal shrinkage represents a good theoretical approach to the improvement of heat bondable fibers, it is believed that this approach does not go far enough.
It will be recognized that while thermal shrinkage per se may be undesirable in a heat bondable fiber, the development of shrinkage force in a nonwoven, brought about subsequent to the creation of interfilamentary bonds may, in fact, be desirable. It is reasonable to assume that shrinkage force, introduced at this time, will not produce any substantial amount of actual shrinkage, but will, rather, remain as a trapped tension in the nonwoven which will enhance such fabric properties as bulk, liveliness, drape and hand.
Accordingly, it is the general object of the present invention to provide improved heat bondable heterofilaments which are useful for the production of nonwoven fabrics, particularly light and medium weight nonwovens, as well as a method for manufacturing such fibers.
It is a more specific object of the invention to provide heat bondable heterofilaments which may be used to produce nonwovents which exhibit minimal thermal shrinkage during thermal bonding but which also exhibit enhanced fabric tensile strength, liveliness, drape, bulk and hand after bonding.
A still more specific object is to provide a heat bondable heterofilament which does not experience substantial shrinkage force, and hence shrinkage, prior to or during thermal bonding, but which does not develop substantial shrinkage force subsequent to the formation of interfilamentary bonds in a nonwoven.
It is a further object to provide a method for manufacturing heat bondable heterofilaments whereby the thermal characteristics of said fiber can be adjusted or altered to meet specified requirements.
Finally, it is an object of the invention to provide nonwovens manufactured from these novel heterofilaments, with said nonwovens being producible at high rates and with modest energy consumption and having enhanced properties.